Transcript Chapter 1

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Chapter 9
The Nucleus, Radioactivity, and
Nuclear Medicine
Denniston
Topping
Caret
5th Edition
9.1 Natural Radioactivity
• Radioactivity - process by which atoms
emit energetic particles or rays
• Radiation - the particles or rays emitted
– comes from the nucleus
• Nuclear symbols - what we use to designate
the nucleus
– Atomic symbol
– Atomic number
– Mass number
9.1 Natural Radioactivity
Nuclear Symbols
mass number
number of
protons and
neutrons
11
5
B
atomic symbol
atomic number
number of protons
9.1 Natural Radioactivity
Writing Nuclear Symbols
11
5
B
• This defines an isotope of boron
• In nuclear chemistry, often called a
nuclide
• This is not the only isotope of boron
– boron-10 also exists
– How many protons and neutrons does
boron-10 have?
• 5 protons, 5 neutrons
9.1 Natural Radioactivity
Three Isotopes of Carbon
• Each nucleus contains the same number of protons
• Only the number of neutrons is different
• With different numbers of neutrons the mass of
each isotope is different
9.1 Natural Radioactivity
Unstable Isotopes
• Some isotopes are stable
• The unstable isotopes are the ones that produce
radioactivity
• To write nuclear equations we need to be able to
write the symbols for the isotopes and the
following:
– alpha particles
– beta particles
– gamma rays
9.1 Natural Radioactivity
Alpha Particles
• Alpha particle (a) - 2 protons, 2 neutrons
• Same as He nucleus (He2+)
• Slow moving, and stopped by small
barriers
• Symbolized in the following ways:
4
2
He
2
4
2
He α
4
2
α
9.1 Natural Radioactivity
Beta Particles
• Beta particles (b) - fast-moving electron
• Emitted from the nucleus as a neutron, is
converted to a proton
• Higher speed particles, more penetrating
than alpha particles
• Symbolized in the following ways:
0
1
e
0
-1
β
β
9.1 Natural Radioactivity
Gamma Rays
• Gamma rays (g) - pure energy
(electromagnetic radiation)
• Highly energetic
• The most penetrating form of radiation
• Symbol is simply…
g
9.1 Natural Radioactivity
Properties of Alpha, Beta, and
Gamma Radiation
• Ionizing radiation - produces a trail of ions
throughout the material that it penetrates
• The penetrating power of the radiation
determines the ionizing damage that can
be caused
• Alpha particle < beta particle < gamma rays
9.2 Writing a Balanced
Nuclear Equation
• Nuclear equation - used to represent
nuclear change
• In a nuclear equation, you do not balance
the elements, instead...
– the total mass on each side of the reaction
arrow must be identical
– the sum of the atomic numbers on each side of
the reaction arrow must be identical
9.2 Writing a Balanced
Nuclear Equations
Alpha Decay
238
92
U
238
=
Th  He
234
90
234
4
2
+
4
mass number
92
=
90
+
atomic number
2
9.2 Writing a Balanced
Nuclear Equations
Beta Decay
• Upon decomposition, nitrogen-16
produces oxygen-16 and a beta particle
• In beta decay, one neutron in nitrogen16 is converted to a proton and the
electron, the beta particle is released
16
7
N O  e
16
8
0
-1
9.2 Writing a Balanced
Nuclear Equations
Gamma Production
• Gamma radiation occurs to increase the
stability of an isotope
– The energetically unstable isotope is called a
metastable isotope
• The atomic mass and number do not
change
• Usually gamma rays are emitted along
with alpha or beta particles
T c T c  g
99 m
43
99
43
9.2 Writing a Balanced
Nuclear Equations
Predicting Products of Nuclear
Decay
• To predict the product, simply remember
that the mass number and atomic number
are conserved
239
92
UX e
0
-1
• What is the identity of X?
239
Np
93
9.3 Properties of Radioisotopes
Nuclear Structure and Stability
• Binding energy - the energy that holds the
protons, neutrons, and other particles
together in the nucleus
• Binding energy is very large
• When isotopes decay (forming more stable
isotopes) binding energy is released
9.3 Properties of
Radioisotopes
Stable Radioisotopes
Important factors for stable isotopes
– Ratio of neutrons to protons
– Nuclei with large number of protons (84 or more)
tend to be unstable
– The “magic numbers” of 2, 8, 20, 50, 82, or 126 help
determine stability – these numbers of protons or
neutrons are stable
– Even numbers of protons or neutrons are generally
more stable than those with odd numbers
– All isotopes (except 1H) with more protons than
neutrons are unstable
9.3 Properties of
Radioisotopes
Half-Life
• Half-life (t1/2) - the time required for
one-half of a given quantity of a
substance to undergo change
• Each radioactive isotope has its own
half-life
– Ranges from a fraction of a second to a
billion years
– The shorter the half-life, the more unstable
the isotope
9.3 Properties of
Radioisotopes
Half-Lives of Selected
Radioisotopes
9.3 Properties of
Radioisotopes
Decay Curve for the Medically
Useful Radioisotope Tc-99m
9.3 Properties of
Radioisotopes
Predicting the Extent of
Radioactive Decay
A patient receives 10.0 ng of a radioisotope with a half-life
of 12 hours. How much will remain in the body after 2.0
days, assuming radioactive decay is the only path for
removal of the isotope from the body?
• Calculate n, the number of half-lives elapsed
using the half-life as the conversion factor
n = 2.0 days x 1 half-life / 0.5 days = 4 half lives
• Calculate the amount remaining
10.0 ng
5.0 ng 2.5 ng 1.3 ng 0.63 ng
1st half-life
2nd half-life 3rd half-life 4th half-life
• 0.63 ng remain after 4 half-lives
9.4 Nuclear Power
Energy Production
E = mc2
• Equation by Albert Einstein shows the connection
between energy (E) and mass (m)
• c is the speed of light
• The equation shows that a very large amount of
kinetic energy can be formed from a small amount
of matter
– Release this kinetic energy to convert liquid water into
steam
– The steam drives an electrical generator producing
electricity
9.4 Nuclear Power
Nuclear Fission
• Fission (splitting) - occurs when a heavy
nuclear particle is split into smaller nuclei
by a smaller nuclear particle
1
0
n
235
92
92
141
1
U  236
U

Kr

Ba

3
92
36
56
0 n  energy
•Accompanied by a large amount of
energy
•Is self-perpetuating
•Can be used to generate steam
9.4 Nuclear Power
Fission of Uranium-235
• Chain reaction - the reaction sustains itself
by producing more neutrons
9.4 Nuclear Power
Representation of the “Energy
Zones” of a Nuclear Reactor
• A nuclear power plant uses a fissionable material
as fuel
–
–
–
–
Energy released by the fission heats water
Produces steam
Drives a generator or turbine
Converts heat to electrical energy
9.4 Nuclear Power
Nuclear Fusion
• Fusion (to join together) - combination of two
small nuclei to form a larger nucleus
• Large amounts of energy is released
• Best example is the sun
• An Example: 2 H  3H  4 He  1n  energy
1
1
2
0
• No commercially successful plant exists in U.S.
9.4 Nuclear Power
Breeder Reactors
• Breeder reactor - fission reactor that
manufactures its own fuel
• Uranium-238 (non-fissionable) is
converted to plutonium-239 (fissionable)
• Plutonium-239 undergoes fission to
produce energy
9.5 Radiocarbon Dating
• Radiocarbon dating - the estimation of the
age of objects through measurement of
isotopic ratios of carbon
– Ratio of carbon-14 and carbon-12
• Basis for dating:
– Carbon-14 (a radioactive isotope) is
constantly being produced by neutrons from
the sun
14
7
N n C H
1
0
14
6
1
1
9.5 Radiocarbon Dating
Radiocarbon Dating
• Living systems are continually taking in
carbon
– The ratio of carbon-14 to carbon-12 stays
constant during its lifetime
• Once the living system dies, it quits taking
in the carbon-14
– The amount of carbon-14 decreases according
14
14
0
to the reaction:
C N e
6
7
-1
• The half-life of carbon-14 is 5730 years
– This information is used to calculate the age
9.6 Medical Applications of
Radioactivity
• Modern medical care uses the
following:
– Radiation in the treatment of cancer
– Nuclear medicine - the use of
radioisotopes in the diagnosis of medical
conditions
9.6 Medical Applications of
Radioactivity
Cancer Therapy Using Radiation
• Based on the fact that high-energy
gamma rays cause damage to
biological molecules
• Tumor cells are more susceptible than
normal cells
• Example: cobalt-60
• Gamma radiation can cure cancer, but
can also cause cancer
9.6 Medical Applications of
Radioactivity
Nuclear Medicine
• The use of isotopes in diagnosis
• Tracers - small amounts of radioactive
substances used as probes to study internal
organs
• Nuclear imaging - medical techniques involving
tracers
• Example:
– Iodine concentrates in the thyroid gland
– Using radioactive 131I and 125I will allow the study of
how the thyroid gland is taking in iodine
9.6 Medical Applications of
Radioactivity
Tracer Studies
• Isotopes with short half-lives are preferred for
tracer studies. Why?
– They give a more concentrated burst
– They are removed more quickly from the body
• Examples of imaging procedures:
– Bone disease and injury using technetium-99m
– Cardiovascular disease using thallium-201
– Pulmonary disease using xenon-133
9.6 Medical Applications of
Radioactivity
Making Isotopes for Medical
Applications
• Artificial radioactivity - a normally stable,
nonradioactive nucleus is made radioactive
• Made in two ways:
• In core of a nuclear reactor
• In particle accelerators – small nuclear
particles are accelerated to speeds
approaching the speed of light and slammed
into another nucleus
9.6 Medical Applications of
Radioactivity
Examples of Artificial
Radioactivity
197
79
Au  n 
1
0
198
79
Au
• Tracer in the liver
• Used in the diagnosis of Hodgkin’s
disease
66
30
Zn  p  Ga
1
1
67
31
9.6 Medical Applications of
Radioactivity
Preparation of Technetium-99m
• Some isotopes used in nuclear medicine have
such a short half-life that they need to be
generated on site
•
99mTc
has a half-life of only 6 hours
99
42
Mo 
Tc e
99m
43
0
-1
9.7 Biological Effects of
Radiation
Radiation Exposure and Safety
The Magnitude of the Half-Life
• Isotopes with short half-lives have one major
disadvantage and one major advantage
– Disadvantage: larger amount of radioactivity per
unit time
– Advantage: if accident occurs, reaches
background radiation levels more rapidly
9.7 Biological Effects of
Radiation
Radiation Exposure and Safety
Shielding
• Alpha and beta particles need a low
level of shielding: lab coat and gloves
• Lead, concrete or both are required for
gamma rays
Distance from the Radioactive Source
• Doubling the distance from the source
decreases the intensity by a factor of 4
9.7 Biological Effects of
Radiation
Radiation Exposure and Safety
Time of Exposure
• Effects are cumulative
Types of Radiation Emitted
• Alpha and beta emitters are generally less
hazardous then gamma emitters
Waste Disposal
• Disposal sites are considered temporary
9.8 Measurement of Radiation
Nuclear Imaging
• Isotope is administered
• Isotope begins to concentrate in the organ
• Photographs (nuclear images) are taken at
periodic intervals
• Emission of radioactive isotope creates the
image
9.8 Measurement of
Radiation
Computer Imaging
• Computers and television are coupled
• Gives a continuous and instantaneous
record of the voyage of the isotope
throughout the body
– Gives increased sensitivity
– CT scanner is an example
9.8 Measurement of
Radiation
The Geiger Counter
• Detects ionizing radiation
• Has largely been replaced by more
sophisticated devices
9.8 Measurement of
Radiation
Film Badges
• A piece of photographic film that is
sensitive to energies corresponding to
radioactive emissions
• The darker the film, when developed,
the longer the worker has been
exposed
9.8 Measurement of
Radiation
Units of Radiation Measurement
The Curie
• The amount of radioactive material
that produces 3.7 x 1010 atomic
disintegrations per second
• Independent of the nature of the
radiation
9.8 Measurement of
Radiation
Units of Radiation Measurement
The Roentgen
• The amount of radiation needed to
produce 2 x 109 ion pairs when passing
through one cm3 of air at 0oC
• Used for very high energy ionizing
radiation only
9.8 Measurement of
Radiation
Units of Radiation Measurement
Rad - Radiation absorbed dosage
• The dosage of radiation able to transfer
2.4 x 10-3 cal of energy to one kg of
matter
• This takes into account the nature of the
absorbing material
9.8 Measurement of
Radiation
Units of Radiation Measurement
The Rem
• Roentgen Equivalent for Man
• Obtained by multiplication of the rad by a
factor called the relative biological effect (RBE)
• RBE = 10 for alpha particles
• RBE = 1 for beta particles
• Lethal dose (LD50) - the acute dosage of
radiation that would be fatal for 50% of the
exposed population
– LD50 = 500 rems